Yankuang ceshi (Nov 2020)
Zircon LA-ICP-MS U-Pb Age and Geochemical Features of the Songlinkou Pluton, Western Sichuan
Abstract
BACKGROUND The Songlinkou pluton is located in the eastern margin of the Songpan-Ganzi orogenic belt, which is adjacent to the pegmatite-type lithium deposit of Jiajika. OBJECTIVES To explore the similarities and differences between Songlinkou pluton and Jiajika mineralized intrusive rocks, and to study geological conditions of mineralization and magma sources of the pluton. METHODS Major and trace elements were determined by X-ray fluorescence spectrometry (XRF) and inductively coupled plasma-mass spectrometry (ICP-MS), respectively. Zircon U-Pb age and Hf isotopes were analyzed by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) and multiple collector-inductively coupled plasma-mass spectrometry (MC-ICP-MS), respectively. RESULTS The Songlinkou pluton contained diorite enclaves, and has SiO2 contents of 56.56%-68.99%, total alkali of 3.78%-6.82%, K2O/Na2O of 1.02-1.93, and Rittmann indices of 1.01-1.93. The rock has relative high Mg# values (Mg#=46.73-61.27). The rock belongs to the high-potassium calcium-alkaline type I granite series showed light rare earth enrichment with LREE/HREE ratios of 2.67-8.31, LaN/YbN of 2.11-9.74. All samples have strongly negative Eu anomaly. It was rich in LILE, and depleted in Ta, Nb, P and Ti. The zircon U-Pb results of the granodiorite and monzogranite were 212.6±1.0Ma (MSWD=0.55) and 222.4±1.1Ma (MSWD=0.39), respectively. The εHf(t) of zircon in monzogranite ranged from -9.09 to -6.86, and TDMC ranged from 1524.874Ma to 1666.002Ma. CONCLUSIONS The Songliankou pluton is produced by the partial melting of the middle Proterozoic Yangtze craton basement mixed with the enriched upper mantle. The Jiajika S-type granite is formed by partial melting of the upper crust, and is different from Songliankou pluton in terms of magma sources, evolution and mineralization tectonic conditions; this may be the main reason why the Songlinkou pluton does not have lithium rare metal mineralization.
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